Ceramic to metal seal

ABSTRACT

This disclosure is directed to metal to refractory seals wherein a ductile 50 atomic percent alloy of a group IVb metal with a group VIII metal of the same period is used to braze ceramic to metal. A preferred example is sealing Tantalum or Niobium to alumina with a 50 a/o Ti-Ni alloy braze.

United States Patent [111 4,

[72] Inventors Russell]. Hill [56| References Cited 20 Marie Drive,R.F.C. N0. 3, Wilmington, UNITED STATES PATENTS Mam. 01887; Rowland M.Cannon Jr. 151 Highland 2,857,663 10/1958 Beggs 29/473 1 2,859,51211/1958 DljkSlElllUlS et al. 29/473.l Ave., Arlington, Mass. 02174 H PPNo 748,227 3,091,028 5/1963 Westbrook et al 29/473.1 [22] Filed July 29968 3,395,993 8/1968 Brlstow 29/473.1 X [45] Patented July 27, 1971Primary Examiner-John F. Campbell Assistant Examiner-Ronald .l Shore 4Att0rneysChar1es M. Hogan and Abraham Ogman [54] CERAMIC T0 METAL SEAL 8Claims, No Drawings [52] US. Cl 29/473.l, ABSTRACT: This disclosure isdirected to metal to refractory 29/ 1 95, 29/504 seals wherein a ductile50 atomic percent alloy of a group 1Vb [51] lnt.Cl. ..B23k 31/02 metalwith a group V111 metal of the same period is used to [501 Field of29/473.1, braze ceramic to metal. A preferred example is sealingTantalum or Niobium to alumina with a 50 a/a Ti-Ni alloy braze.

CERAMIC TO METAL SEAL The present invention relates to the bonding ofnonmetallic refractory members to metal members, and more particularlyto a temperature resistant, oxidation resistant metal to ceramic sealcapable of use in cesium environments.

The problems involved in obtaining satisfactory ceramic to metal sealsare well known, and various brazing alloys have been suggested byworkers in the art, as in, for example, US. Pats. Nos. 3,091,028 and2,857,663. Such seals are important to satisfactory operation of highpower electron tube devices and are notably so in devices employingmetal vapors or liquid therein such as are devices and thermionicconverters.

A common prior art method of making such seals involves first placing alayer of metal on the surface of the ceramic body or member then brazingthe metal body or member to the metallized ceramic with the aid of afusible metallic shim therebetween. Any of the several diverse metal tometal interfaces present in the so sealed structure may have a brittleintermetallic phase in the metal junction due to interaction betweenmetal, braze, and metallized ceramic. Besides the high failure rate, thejoint or seal as a whole may be weak due to the various intermetallicphases present. Eliminating the metallizing step and brazing directly tothe ceramic body offers promise for more facile sealing techniques, evenbetter seals. The braze metal must then be some material which reactsstrongly with the ceramic in order to achieve the desired bond. Yet, thehigh chemical reactivity of such a braze metal may cause the braze alsoto react strongly with the metal member. Thus, ifa pure metal braze isemployed, the solution resulting from dissolution of the metal body andthe braze metal has a progressively lower melting point than the purebraze metal and substantial, even complete, dissolution of the metalbody can result if the completed seal is subjected to high operatingtemperatures. On the other hand, if a eutectic braze is employed, themetal from the metal body which dissolves therein often forms a brittleintermetallic phase between itself and one or both of the brazeconstituents.

1n any event, an upper operating limit of about 500 C. is commonly setfor the ultimate equipment sealed by conventional brazing alloys and theheretofore employed ceramic to metal sealing techniques.

An object of the present invention is to provide an improved bond forjoining nonmetallic refractory bodies to metallic members.

A further object of the invention is to provide a ceramic to metal sealhaving good compatibility with a cesium environ ment and operation atelevated temperatures.

Still another object of the invention is to provide a high qualityceramic to metal seal employing a ductile metal braze.

Further objects and the advantages of the present invention will beapparent from the description thereof which follows.

Briefly stated, the practice of the present invention comprises formingthe seal with a 50 atomic percent alloy of one member selected from thegroup lVb metals and the other member selected from the group Vlllmetals of the same period. More specifically, the alloys contemplatedfor the braze material are the 50 a/o alloys of: titanium with iron,cobalt, nickel or mixtures thereof; hafnium with osmium, iridium,platinum, or mixtures thereof; zirconium with rhodium, ruthenium,palladium, or mixtures thereof.

Most significant in terms of the ceramic to metal seal is that thisgroup of 50 atomic percent alloys or intermetallic compounds are ductileand have a significant homogeneity range on either side of the 50 (1/0.They remain ductile within this range of homogeneity. Thus, the Ti-Nisystem has a homogeneity range of 46-53 /0 Ti. The other systems havesimilar, but not necessarily identical homogeneity ranges. For furtherdescription of the alloys per se reference is made to a series ofarticles by F. B. Wang or F. E. Wang et al. in Journal of AppliedPhysics, Vol. 36, p. 3232 (1965); Vol. 38, p. 822 (1967); and Vol. 29,p. 2192 (1968). Attention is directed also to US. Pat. No. 3,174,851 fordescription of the 50 a/a Ti-Ni alloy.

While reference has been made above to 50 a/o, the ductile alloy brazescontemplated for practice of this invention may be of any specificcomposition within their homogeneity range. Therefore, within thecontext of this invention a general reference to these alloy brazes as50 a/o ductile alloys should be taken as a reference to include theentire range of homogeneity. As a practical matter the braze alloycomposition should be held within somewhat narrower limits than theentire homogeneity range, 50 a/trt2 a/o being preferred.

When the lVb-Vlll ductile 50 a/o alloys are used as brazes to joinceramic bodies to metal members, they perform in a beneficial andperhaps unique manner. The molten alloy reacts with the ceramic to forma hermetic joint, eliminating need for preliminary metallizing of theceramic. Moreover, the braze reacts only slightly with the metal memberin the seal assembly. Any intermetallic reaction products between thebraze alloy and the metal member are not precipitated as a brittleintermetallic phase. After the seal has been formed no subsequentconditioning steps are needed. These ductile 50 a/o alloy brazes havegeneral applicability to the many refractory ceramics usually joined tometal members, including for example, alumina, zirconia, magnesiayttria, sapphire. They have, also, general applicability to the metalsusually joined to ceramics for electronic uses, including for example,tantalum, niobium, the group Vlll metals. lmportantly, they can be usedat elevated temperatures, e.g. to 800 C.

One exemplary instance of a preferred embodiment of practice accordingto the invention is formation of a seal between a niobium member or atantalum member and a high' purity alumina body with a ductile 50 (1/0alloy of titanium-nickel.

Use of these ductile 50 (1/0 alloy brazes permits seal fabrication byrelatively uncomplicated techniques. According to one method, theceramic body and the metal member to be joined are juxtaposed with ashim or wire of the chosen interrnetallic alloy placed between them.This assembly is then heated, eg in a vacuum furnace or by radiofrequency induction heating in an argon atmosphere to the melting pointof the chosen alloy, at which point the molten alloy reacts with andwets the ceramic body and also brazes to the metal member. The brazematerial remains as a single phase ductile intermetallic alloy.Thereafter the assembly is cooled as rapidly as the ceramic will allow.The joint is ready for use without further conditioning. It is airtight,heat and oxidation resistant and stable for use over extended periods oftime; it may be employed in a cesium environment, I

For further understanding of the invention more detailed specificexamples of the practice thereof is now presented.

A nickel titanium alloy of exactly 50 a/o employed as the braze alloyhad the following properties:

Density 6.45 g./cc. Melting point I250 C. Electrical resistivity flcm.

(room temperature) Expansion coefficient IOAXIO" C." Ultimate tensilestrength l40,000 p.a.i. Yield strength 8l,000 p,s.i. Young's Modulus 11X10 p.s.i.

Tensile elongation up to 15 percent A washer (0.005 inches thick) of thealloy was placed at the bottom of a tantalum cup and a high purityalumina tube was placed on top the washer. The so assembled cup, washerand tube was heated inductively in a stream of commercial grade argon toa temperature just above 1250" C. to melt the washer. The assembly wasthen cooled in the argon stream and removed, with the whole heating,melting and cooling process taking about 5 minutes. Examination of thecooled assembly showed that very little dissolution of the tantalum hadtaken place during the brief period the braze was molten and that anairtight satisfactory seal was formed.

In the same fashion a 50 all) hafnium-iridium hraze alloy washer formeda good seal between a tantalum cup and an alu mina tube.

Sealed assemblies, sealed by the 50 a/o 'l'i-Ni, braze alloy in themanner described above were fully fabricated and tested for temperatureresistance in the presence of cesium. They proved satisfactory atelevated temperatures up to 800 C.

In the same fashion a tantalum tube was brazed to magnesia with theabove 50 a/o Ti-Ni alloy. In this instance the differential expansionbetween magnesia and tantalum caused the magnesia to crack upon cooling,but the seal itself appears satisfactory.

What we claim is:

l. A metal to ceramic seal which consists essentially of a ductilebrazing alloy interposed between the metal and ceramic, said alloyconsisting essentially of a single phase ductile alloy within the rangeof 48 a/52 a/o of titanium with a member selected from the groupconsisting of Fe, and Co and mixtures thereof,

2. A seal as in claim 1 wherein said ceramic is alumina, said metal isselected from the group consisting of Tantalum and Niobium and said 50(1/0 brazing alloy is a Ti-Fe or Ti-Co alloy having a composition rangeof 50 a/oi 2 a/o.

3. A metal to ceramic seal which consists essentially of a ductilebrazing alloy interposed between the metal and ceramic, said alloyconsisting essentially of a single phase ductile alloy within the rangeof 48 a/o52 a/o of zirconium with a member selected from the groupconsisting of Ru, Rh and Pd and mixtures thereof.

4. A metal to ceramic seal which consists essentially of a ductilebrazing alloy interposed between the metal and ceramic, said alloyconsisting essentially of a single phase ductile alloy within the rangeof 48 a/o-52 all) of hafnium with a member selected from the groupconsisting of Os, lr, Pt and mixtures thereof.

5. A method of bonding a metal member and a nonmetallic refractory bodywhich consists essentially of interposing therebctween a preform of asingle phase ductile alloy within the range of 48 a/o-52 a/o of titaniumwith a member selected from the group consisting of Fe and Co, andmixtures thereof, then melting only said alloy under nonoxidizingconditions, and thereafter cooling the assembly whereby said alloy formsa tight high temperature resistant bond between the metal member and therefractory body.

6. The method of claim 5 wherein said ceramic is alumina, saidmetal isselected from the group consisting of Tantalum and Niobium and said 50a/o brazing alloy is a Ti-Fe or Ti-Co alloy having a composition rangeof 50 ale: 2 a/o.

7. A method of bonding metal member and a nonmetallic refractory bodywhich consists essentially of interposing therebetween a preform of asingle phase ductile alloy within the range of 48 a/o-52 a/o ofzirconium with a member selected from the group consisting of Ru, Rh, Pdand mixtures thereof, then melting only said alloy under nonoxidizingconditions, and thereafter cooling the assembly whereby said alloy formsa tight high temperature resistant bond between the metal member andrefractory body.

8. A method of bonding a metal member and a nonmetallic refractory bodywhich consists essentially of interposing therebetween a preform of asingle phase ductile alloy within the range of 48 a/o52 a/a of hafniumwith a member selected from the group consisting of Os, Ir, Pt andmixtures thereof, then melting only said alloy under nonoxidizingconditions, and thereafter cooling the assembly whereby said alloy formsa tight high temperature resistant bond between the metal member and therefractory body.

atent No. 9 9 Dated July 27, 1971 Inventor) Russell J. Hill and RowlandM. Cannon It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 58, for "10. 4x10 0 read--- 10.4- c

and Column 3, line 1 through line 3 should read---ln the same fashion a.50 a/o zirconium-palladium braze alloy Washer and a 50 a/ohafniumiridium braze alloy washer formed a good seal between a tantalumcup and an alumina tube.

Signed and sealed this 12th day of September 1972.

(SEAL) Attest:

EDWARD M.F'LETCHER,JR. ROBERT GOT'ISCHALK Attesting Officer Commissionerof Patents

2. A seal as in claim 1 wherein said ceramic is alumina, said metal isselected from the group consisting of Tantalum and Niobium and said 50a/o brazing alloy is a Ti-Fe or Ti-Co alloy having a composition rangeof 50 a/o + or - 2 a/o.
 3. A metal to ceramic seal which consistsessentially of a ductile brazing alloy interposed between the metal andceramic, said alloy consisting essentially of a single phase ductilealloy within the range of 48 a/o- 52 a/o of zirconium with a memberselected from the group consisting of Ru, Rh and Pd and mixturesthereof.
 4. A metal to ceramic seal which consists essentially of aductile brazing alloy interposed between the metal and ceramic, saidalloy consisting essentially of a single phase ductile alloy within therange of 48 a/o- 52 a/o of hafnium with a member selected from the groupconsisting of Os, Ir, Pt and mixtures thereof.
 5. A method of bonding ametal member and a nonmetallic refractory body which consistsessentially of interposing therebetween a preform of a single phaseductile alloy within the range of 48 a/o- 52 a/o of titanium with amember selected from the group consisting of Fe and Co, and mixturesthereof, then melting only said alloy under nonoxidizing conditions, andthereafter cooling the assembly whereby said alloy forms a tight hightemperature resistant bond between the metal member and the refractorybody.
 6. The method of claim 5 wherein said ceramic is alumina, saidmetal is selected from the group consisting of Tantalum and Niobium andsaid 50 a/o brazing alloy is a Ti-Fe or Ti-Co alloy having a compositionrange of 50 a/o + or - 2 a/o.
 7. A method of bonding metal member and anonmetallic refractory body which consists essentially of interposingtherebetween a preform of a single phase ductile alloy within the rangeof 48 a/o- 52 a/o of zirconium with a member selected from the groupconsisting of Ru, Rh, Pd and mixtures thereof, then melting only saidalloy under nonoxidizing conditions, and thereafter cooling the assemblywhereby said alloy forms a tight high temperature resistant bond betweenthe metal member and refractory body.
 8. A method of bonding a metalmember and a nonmetallic refractory body which consists essentially ofinterposing therebetween a preform of a single phase ductile alloywithin the range of 48 a/o- 52 a/o of hafnium with a member selectedfrom the group consisting of Os, Ir, Pt and mixtures thereof, thenmelting only said alloy under nonoxidizing conditions, and thereaftercooling the assembly whereby said alloy forms a tight high temperatureresistant bond between the metal member and the refractory body.